In the frame of the safety assessment of IFMIF(International Fusion Materials Irradiation Facility), an analysis has been developed, aimed at evaluating an unavailability figure of the target system, and eventually at evaluating the expected performance of the plant, in terms of continuous and reliable operation. Starting points of the analysis have been the identification of functions relevant for plant availability and relationships between plant systems and functions and the definition of relationships among the systems themselves. Fault tree technique has been adopted to address the topic: despite the early stage of the design - at present many systems are not yet definitely established - systems boundaries and interfaces have been identified as well as the dependent failures between systems and components. Due to the novelty of the plant and its prototypical character, reliability data introduced in the numerical simulation underwent an accurate screening process among the current available databases, sometimes requiring the expert judgement assessment. Finally an uncertainty, importance and sensitivity analysis has been performed in order to add credit to the achieved results and to highlight reliability-critical systems and components: results are analysed to assess the compliance with the plant availability requirements and design criteria. Risk Spectrum code has been utilised for the system unavailability quantification.
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